The present invention regards a system for chemical conversion of natural gas or another suitable fossil fuel to synthetic hydrocarbons (syncrude). In particular, the present invention regards a system for optimising the production of synthetic hydrocarbons.
Known processes for conversion of natural gas or other fossil fuels to synthetic hydrocarbons comprise two steps. First, the natural gas or other fossil fuel is converted to synthesis gas, i.e. a mixture consisting predominantly of hydrogen and carbon monoxide, as well as some CO2, which in a second step is converted to synthetic hydrocarbons through the so-called Fischer-Tropsch (FT) synthesis. The synthetic hydrocarbon product normally consists of higher hydrocarbons, i.e. pentane and higher compounds (C5+). The process may also include an additional step in which the synthetic hydrocarbon crude product is upgraded to final products.
Synthesis gas for production of synthetic hydrocarbons is normally produced by steam reforming or partial combustion, or a combination of these two reactions. The water gas shift reaction also plays an important part in the production of synthesis gas. These reactions may be written as follows:
The Fischer-Tropsch synthesis for producing synthetic hydrocarbons may be written as follows:
where [xe2x80x94CH2xe2x80x94] is the basic building block for the hydrocarbon molecules. The FT synthesis is highly exothermic, which leads to heat transfer being a significant factor in the design of an FT reactor.
An important parameter for determining the theoretical maximum yield of synthetic hydrocarbons is the stochiometric number SN, defined as:
SN=(H2xe2x80x94CO2)/(CO+CO2)xe2x80x83xe2x80x835)
Theoretically, the yield of synthetic hydrocarbons is at its highest when SN=2.0 and CO does not react further to form CO2 via the water gas shift reaction (equation 3). In this case, the H2/CO ratio will be equal to SN, i.e. 2.0, which theoretically gives the highest yield of synthetic hydrocarbons in accordance with equation 4. In practice however, the production of synthesis gas will always involve the water gas shift reaction to a certain degree, so that the CO yield, and thus also the synthetic hydrocarbon yield, becomes somewhat lower.
Further, the maximum yield of synthetic hydrocarbons is in reality achieved at a somewhat lower H2/CO ratio, typically around 1.6-1.8. At an H2/CO ratio of 2.0 or more, the synthetic hydrocarbon yield will be reduced due to the formation of more methane and other lower hydrocarbons (C4xe2x88x92), which are normally undesirable products.
The preferred technology for producing synthetic hydrocarbons from synthesis gas is non-catalytic partial oxidation (POX) or autothermal reforming (ATR), in which partial combustion is combined with adiabatic catalytic steam reforming (equation 1) in the same reactor unit.
Another technology is combined reforming with a tubular catalytic steam reformer followed by an ATR.
A desired H2/CO ratio is achieved by running the synthesis gas reactor with a combination of a low steam/carbon ratio (S/C) and a high temperature, in addition to recirculating part of the CO2-rich tail gas from the FT synthesis to the synthesis gas reactor in order to limit the water gas shift activity (equation 3). In this manner, the H2/CO ratio will approach the achieved value of SN.
The drawback of the known techniques for producing synthetic hydrocarbons is low carbon efficiency in comparison with the theoretical achievement. The carbon efficiency is defined as the relationship between the total amount of carbon in the produced crude product of synthetic hydrocarbons and the total amount of carbon in the natural gas feed. As such, the carbon efficiency is a measure of how much of the carbon in the feed actually ends up in the final product, and how much ends up as CO2. A plant with low carbon efficiency gives a low product yield, a large CO2 emission and thus an environmental problem.
As mentioned, catalytic autothermal reforming (ATR) and non-catalytic partial oxidation (POX) are the preferred technologies for production of synthesis gas for the FT synthesis. By using natural gas as a feed, these technologies produce a synthesis gas with an SN value typically in the range 1.6 to 1.8, which gives the highest yield of synthetic hydrocarbons locally in the FT reactor. However the SN value is lower than 2.0, which for the plant as a whole implies a lower carbon efficiency than that which may theoretically be achieved, due to a hydrogen deficiency.
Combined reforming, which normally takes place in a tubular catalytic steam reformer followed by a secondary reformer with an oxygen feed, is capable of producing synthesis gas with an SN value of 2.0, which should theoretically give the highest carbon efficiency in the plant for production of synthetic hydrocarbons. The real carbon efficiency will however not be higher than that which is achieved by use of POX or ATR, due to the higher degree of recirculation of tail gas to the synthesis reaction that is required in order to restrict a greater water gas shift activity than in the ATR as a result of the higher S/C ratio, and due to a lower yield of the desired higher synthetic hydrocarbons at this SN value.
It is thus an object of the present invention to provide an improved method for conversion of natural gas or other fossil fuels to higher hydrocarbons, in which the above mentioned drawbacks of the known techniques have been overcome.
According to the present invention, this is achieved by a method for conversion of natural gas or other fossil fuels to higher hydrocarbons, which comprises the steps of:
a) reacting natural gas with steam and oxygenic gas in at least one reforming zone in order to produce a synthesis gas that consists primarily of H2 and CO, in addition to some CO2;
b) lead said synthesis gas to a Fischer-Tropsch reactor in order to produce a crude synthesis stream consisting of lower hydrocarbons, higher hydrocarbons, water, and unconverted synthesis gas;
c) separating said crude synthesis stream in a recovery zone, into a crude product stream that primarily contains lower hydrocarbons, higher hydrocarbons, a water stream and a tail gas stream that mainly contains the remaining constituents; characterised in that the method also comprises the steps of;
d) steam reforming at least part of the tail gas in a separate steam reformer;
e) introducing the reformed tail gas into the gas stream before this is fed into the Fischer-Tropsch reactor.
xe2x80x9cLower hydrocarbonsxe2x80x9d refers to C1-C4 hydrocarbons. xe2x80x9cHigher hydrocarbonsxe2x80x9d refers to C5+ hydrocarbons.
It is preferable for the steam reforming in step d) to take place at conditions that favour the conversion of CO2 to CO by reversible water gas shift reaction.
Moreover, it is preferable to also hydrogenate that part of the tail gas that is steam reformed, in order to saturate any unsaturated hydrocarbons prior to step d).
In a preferred embodiment, natural gas is fed to the steam reformer in step d) together with the tail gas feed.
In a preferred embodiment, the reformed tail gas is introduced into the gas stream after step a), but before step b).
In another preferred embodiment, the reformed tail gas is introduced into the gas stream before step a).
It is also preferred that part of the reformed tail gas be introduced into the gas stream before step a) and part of it be introduced after step a) but before step b).
Use of the present method has several advantages over previously known techniques.
By reforming and recirculating the tail gas, it becomes possible to:
Increase the SN value from typically 1.6-1.8 for an ATR to approximately 2.0.
Maintain or increase the CO yield, so that the H2/CO ratio approaches the SN value.
Achieve an H2/CO ratio of less than 2.0 locally at the inlet to the FT reactor, which gives a higher yield of higher hydrocarbons.
The present method results in higher carbon efficiency and higher thermal efficiency. This gives a desired reduction in the CO2 emission, which is desirable, both for environmental and economic reasons. The oxygen consumption by the present method is lower than in the case of conventional plants for production of synthesis gas by use of POX or ATR, which entails reduced capital costs and lower power consumption.
It is also possible to achieve operational benefits such as increased stability by the oxygen fired synthesis gas reactor operating at a somewhat lower output temperature than that which is the case when using previously known technology. The increased methane content (lower conversion of natural gas) caused by this will be reformed in the tail gas reformer.
By eliminating the recirculation of tail gas to the main section for synthesis gas, it is also possible to economise with regard to the size of the equipment, and thereby to save costs in this section.